It's left ambiguous at the end of the original story. Kitty's mind is restored and they have no way of knowing if they succeeded in saving the future. I really liked that when I first read it, though I'm not sure it would be as effective in the film.

X-Men Trilogy timeline (2013?):
Post-X3 Magneto seeks revenge on humans. He finds a mutant named Forge.
Magneto uses one of Forge's inventions to send Mystique's mind into her 1960's body.
First Class / Dystopian timeline (1963-20xx):
Mystique convinces 1960's Magneto to assassinate JFK and frame it on the X-Men.
Mutants are blamed for the JFK assassination, leading to the mutant oppression.
FAST FORWARD: The last few mutants struggle to fight off the onslaught of sentinels.
Magneto is found held captive in an internment camp and is rescued by the mutants.
Magneto reveals the truth about what he did to change the future, dying shortly after.
Forge invents his machine and sends a mutant back to the 1960's to stop Magneto.
New First Class timeline (1963):
The time-traveler goes and tells the X-Men what Magneto plans to do.
The X-Men and the Brotherhood have a long epic battle as they always do.
The X-Men prevent Magneto from assassinating JFK and having it blamed on them.
JFK is assassinated anyway, but by Lee Harvey Oswald just as history tells it.

I have a sort of movie treatment for Days of Future Past. It's a little convoluted but I think it works out pretty well.

Skip to the end for the short version. Read the long version for explanations and implications.

The movie begins at some point after X-Men: The Last Stand. The cure is widely available and mutant-human peace has been reached with the help of mutant ambassador Hank McCoy. Magneto, on the other hand, is not so happy. His best friend/rival is dead, he feels impotent without his full powers, and worst of all, mutants are now being considered equal to humans, not superior like he believes they should be. He finds a mutant, Forge, who believes in his cause. Forge has invented a machine which allows someone's mind to occupy their body in the past. Magneto uses this to send Mystique (he could go back himself but I don't find that as interesting) back into her 1960s body to help trigger an event that will draw out the divide between mutants and humans forever.

What event the writers would decide to use is anyone's guess, but let's go with the Kennedy Assassination for now. In this new past, Mystique convinces Magneto that they should teach humans a lesson for taking all the credit and not acknowledging the superiority of mutant powers which ultimately prevented nuclear war. They carry out the assassination on JFK: Magneto uses his powers to guide the bullet while Mystique "gets caught" disguised as an X-Man, thus leading humans to blame the X-Men for this tragedy. This in turn leads to the mutant oppression where sentinels are mass produced to hunt down and eliminate the mutant species.

Now in this new timeline anything that occurred in X1-3 essentially does not matter, and yet they didn't have to outright forget the films because the movie will have begun in the X1-3 timeline thus acknowledging all the events that occurred in those films. This means that in the new timeline they can fast forward to the dystopian future and use ANYONE who wants to reprise their roles from X1-3 to fight the sentinels, and for anyone who does not come back, their characters can be assumed dead (they can even show a graveyard scene with their tombstones).

So in this new future the last remaining mutants have banded together to survive. Forge is now a "good guy" who CAN invent his machine but hasn't thought of it yet. It turns out Magneto was one of the first few mutants captured and put into an internment camp. The mutants find him and free him, at which point he reveals the truth about how these events came to be. Forge then (re)creates his machine allowing the mutants to send somebody back to the past to prevent the Brotherhood from carrying out and framing the assassination. For plot-hole purposes, Mystique should have died at some point in this future, otherwise she could possibly go back a second time. Similarly, Magneto would probably have to die before the machine is invented so he can't go back and just not carry out the assassination

Who do the mutants send back to prevent Magneto from assassinating JFK? At this point, it probably doesn't even matter, although it's a perfect opportunity to introduce a new character (or reintroduce an X1-3 character) into the First Class cast. The rest of the movie (should be at least a third of the overall movie, maybe even half) would occur in the 1960's in a new third timeline. In this timeline, there would be a battle between the X-Men and the Brotherhood where the X-Men succeed in preventing Magneto from getting anywhere near JFK. However, what they did not know and therefore could not prevent was the fact that Lee Harvey Oswald, a human, would carry out the assassination anyway as he was preparing to do originally before Magneto changed the past.

Now, only having introduced one new plot-intensive character (Forge), we have made a movie loosely (yes I know, very loosely) based off Days of Future Past which serves as a sequel to First Class while still acknowledging the original X-Men trilogy, but allowing future sequels with the First Class cast to have complete creative freedom with the story.

SHORT VERSION:

X-Men Trilogy timeline (2013?):

Post-X3 Magneto seeks revenge on humans. He finds a mutant named Forge.
Magneto uses one of Forge's inventions to send Mystique's mind into her 1960's body.

First Class / Dystopian timeline (1963-20xx):

Mystique convinces 1960's Magneto to assassinate JFK and frame it on the X-Men.
Mutants are blamed for the JFK assassination, leading to the mutant oppression.
FAST FORWARD: The last few mutants struggle to fight off the onslaught of sentinels.
Magneto is found held captive in an internment camp and is rescued by the mutants.
Magneto reveals the truth about what he did to change the future, dying shortly after.
Forge invents his machine and sends a mutant back to the 1960's to stop Magneto.

New First Class timeline (1963):

The time-traveler goes and tells the X-Men what Magneto plans to do.
The X-Men and the Brotherhood have a long epic battle as they always do.
The X-Men prevent Magneto from assassinating JFK and having it blamed on them.
JFK is assassinated anyway, but by Lee Harvey Oswald just as history tells it.

in the case time travel existed, even if someone goes back to the past and changes a big event, ALL the humans that have existed between past and present wouldnt dissapear. thats like a joke.

Imagine you are in your house, and someone went back to the 70's, and destroyed the whole USA (or wherever you live) with a nuclear bomb, and just on that same second, you and your whole family, neighbors, and all the americans would just dissapear suddenly? doesnt seem 'realistic'.

The alternate universe feels more realistic, even if time travel is a fantastical theme.

So if Bryan and co. go with this route, the characters from the future and the actual future will still be the same, but another universe will be created with totally different citizens, streets, look and all.

The conclussion would be: each time the past is changed, a new alternate universe would be created.

You could seriously fry your brain trying to think about time travel and its implications

Alternate universe does seem like a better solution, but as I've mentioned previously that option kinda means that time travel and all the efforts in the past will do absolutely nothing for the dark-future!mutants who'll just stay in their nightmarish future.

They could leave the question of whether the future is changed or not ambiguous, but if the movie follows the future!mutants throughout then leaving them without a resolution might come off as unsatisfying. Unless they just decide to kill them all off.

You could seriously fry your brain trying to think about time travel and its implications

Alternate universe does seem like a better solution, but as I've mentioned previously that option kinda means that time travel and all the efforts in the past will do absolutely nothing for the dark-future!mutants who'll just stay in their nightmarish future.

They could leave the question of whether the future is changed or not ambiguous, but if the movie follows the future!mutants throughout then leaving them without a resolution might come off as unsatisfying. Unless they just decide to kill them all off.

Ahhh.. but this is what i always wondered... After you changed a present situation you kinda left with two universes, the one that you came from and the Alternate one that was created, from your actions in the present.

Now you have a time machine, that i would assume jumps from one point in time, to another. it doesnt jump to alternate universes... so you would be lost to the world that you came from, as essentially if you tried to return to the future from which you came, wouldnt you just end up in the future of the present universe your in?

You could seriously fry your brain trying to think about time travel and its implications

Alternate universe does seem like a better solution, but as I've mentioned previously that option kinda means that time travel and all the efforts in the past will do absolutely nothing for the dark-future!mutants who'll just stay in their nightmarish future.

They could leave the question of whether the future is changed or not ambiguous, but if the movie follows the future!mutants throughout then leaving them without a resolution might come off as unsatisfying. Unless they just decide to kill them all off.

Depends where they want the films to go next.

If the X1-X3 continuity is erased by changing history, then so is The Wolverine, which takes place after X3. However, as Angamb said, it doesn't seem logical for a whole timeline to just vanish.

But if that X1-X3-DoFP continuity remains as an alternate reality, then we know The Wolverine leads to DoFP and that leads to sheer hell for mutants!

It's head-spinning, brain-frying stuff but it all needs thinking about.

Part of the decision needs to be based on where the X-Men franchise will go next, after DoFP. What story, what setting, what time period, what characters, does Fox want next? That's the key.

In the Wolverine and the X-Men cartoon, the prevention of DoFP led to Age of Apocalypse taking place instead.

Ahhh.. but this is what i always wondered... After you changed a present situation you kinda left with two universes, the one that you came from and the Alternate one that was created, from your actions in the present.

Now you have a time machine, that i would assume jumps from one point in time, to another. it doesnt jump to alternate universes... so you would be lost to the world that you came from, as essentially if you tried to return to the future from which you came, wouldnt you just end up in the future of the present universe your in?

well, time machine is another thing.

a time mach. the same as a mutant power could allow you to travel to other universes, as seen in the comics.

so having said that, you can go and come back to any universe you want. At least if you can use the time machine right

FANTASTIC FOUR"This is our chance to make a difference" - Sue Storm - The Invisible Woman"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic"We could these powers to help people" - Johnny Storm - The Human Torch"You can't fix this, nobody can" - Ben Grimm - The Thing

I saw some pics of him, I like some of his pics, but I kind of hate how he look in Avatar stills and photos. I dont see Cable there. a bit old and I remember seeing him thin, its kind of weird to see an older actor with a thin body but muscular at the same time

Basically Cable will be a dude in his 30s or early 40s but with white hair. They're not gonna go too old, if they do include him. That Liefeld art always made him look older than he is.

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If the person you're seeing ever asks the question "Who is Stan Lee?", promptly kick their ass to the curb.
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Who the **** makes a movie and while planning it is like, "you know what this needs...is some Greg Kinnear."

Post-X3 Magneto seeks revenge on humans. He finds a mutant named Forge.
Magneto uses one of Forge's inventions to send Mystique's mind into her 1960's body.

First Class / Dystopian timeline (1963-20xx):

Mystique convinces 1960's Magneto to assassinate JFK and frame it on the X-Men.
Mutants are blamed for the JFK assassination, leading to the mutant oppression.
FAST FORWARD: The last few mutants struggle to fight off the onslaught of sentinels.
Magneto is found held captive in an internment camp and is rescued by the mutants.
Magneto reveals the truth about what he did to change the future, dying shortly after.
Forge invents his machine and sends a mutant back to the 1960's to stop Magneto.

New First Class timeline (1963):

The time-traveler goes and tells the X-Men what Magneto plans to do.
The X-Men and the Brotherhood have a long epic battle as they always do.
The X-Men prevent Magneto from assassinating JFK and having it blamed on them.
JFK is assassinated anyway, by the Comedian!

Which also further proves the Hollywood double standard. Amy Adams gets flack for being close to 40. Famke Janssen can't return as Jean cause she's pushing 50. People always complaining about the age of the female characters, but the males can be any age.

Stephen Lang is like the safest choice for the role! Thats like saying Jaime Alexander should be Wonder Woman! So no.

And JP, Amy Adams and Famke Jannsen look good for their age.

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FANTASTIC FOUR"This is our chance to make a difference" - Sue Storm - The Invisible Woman"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic"We could these powers to help people" - Johnny Storm - The Human Torch"You can't fix this, nobody can" - Ben Grimm - The Thing

He should play him but I'd rather see Bishop because we have no black peoples. Unless they wanna get really complicated and include both Cable AND Bishop but I can't see that unless they're following up with AoA